1. Field of the Invention
The present invention relates to a protective apparatus and method for protecting a fuel cell from overload or short-circuit failures that may occur in a large capacity fuel cell power supply. Here, the large capacity fuel cell power supply refers to a power system that has a capacity of more than 1 MW, such as a 5 MW or 10 MW fuel cell power supply.
2. Description of the Prior Art
A fuel cell power supply, being different from the other power sources, maintains its output current nearly constant even if an overload or short-circuit failure occurs in the output circuit. This presents a problem that overcurrent protective apparatuses for the other power sources cannot be applied to the fuel cell power supply.
FIG. 1 is a schematic block diagram showing a conventional fuel cell power supply. A current detector (CT) 16, e.g. a current transformer, detects the output current I.sub.F supplied from a fuel cell (FC) 1 to a load (R.sub.L) 7. A controller (CNT) 17 controls a raw-fuel control valve 18 in accordance with the detected current. The raw-fuel control valve 18 supplies required raw-fuel (Q) to a reformer (RF) 20.
In the reformer 20, hydrogen U.sub.I, which is generated by the reforming reaction between methanol and water in the form of mixture, is fed to the fuel cell (FC) 1. The fuel cell 1 converts chemical energy generated by the electrochemical reaction between hydrogen and oxygen into electrical energy having the quantity of electricity of U.sub.E. A part of the unreacted hydrogen in the fuel cell 1 is produced as off gas U.sub.B. The off gas U.sub.B is fed back to the burner of the reformer 20 to be burned to maintain the temperature of the reforming catalyst filled in the reformer 20.
In FIG. 2, a curve A shows the I-V characteristics of the fuel cell 1. Reference character U.sub.I designates the quantity of electricity corresponding to the total quantity of the hydrogen produced by the reforming reaction in the reformer 20. The quantity U.sub.I corresponds to a current I.sub.FS, while the normal rated current I.sub.FN corresponds to the quantity of electricity U.sub.E.
Increasing the load 7 changes the output current and voltage of the fuel cell 1 in accordance with the I-V characteristics of the fuel cell 1. The voltage V.sub.FO is the no-load voltage. The voltage V.sub.FP corresponds to the minimum current flowing into the load 7 from the fuel cell 1, and the line passing this point and the origin defines the minimum load line B. The voltage V.sub.FN is the voltage of the fuel cell 1 in the normal operation, and the line passing this point (I.sub.FN, V.sub.FN) and the origin defines the rated load line C.
If the voltage of the fuel cell 1 decreases lower than V.sub.FN owing to a short-circuit failure or an overload due to the increase of the load 7, that is, if the output current of the fuel cell 1 exceeds I.sub.FN, the voltage will rapidly reduce to V.sub.FS instead of tracing the broken line D which is the extended line of the I-V characteristic line. The line passing the point of (I.sub.FS, V.sub.FS) and the origin is called short-circuit line E. The intersection S of the lines D and E corresponds to the current I.sub.S which will be described later.
Generally, the ratio U.sub.E /U.sub.I is called fuel availability. The availability can be calculated by performing a division between the two after converting the hydrogen quantity U.sub.I into the current I.sub.FS or after converting the current I.sub.FS detected by the current detector 16 into the hydrogen quantity U.sub.I.
The fuel cell power supply is usually operated at the availability of about 0.75-0.8. If a short-circuit failure occurs in such an operating state, the hydrogen to be fed to the reformer 20 as the off gas is completely consumed by the electrochemical reaction in the fuel cell 1.
This makes U.sub.I =U.sub.E, and so I.sub.FS =I.sub.FN /(0.75-0.8)=(1.33-1.25).times.I.sub.FN, thus resulting in a small increase in the overcurrent. In addition, the short-circuit or overload in the fuel cell power supply poses a problem that it causes a gas shortage, which will degrade the fuel cell 1.
In power supplies other than a fuel cell, the short-circuit or overload will cause a current I.sub.S of several times the rated current. This can be easily detected by an ordinary overcurrent protective apparatus, and so the impaired circuit can be readily disconnected in response to the detection result.
A fuel cell power supply, however, cannot take quick action to protect the fuel cell 1, because the excess part of the overcurrent is very small: the overcurrent of only 1.33-1.25 times an rated current occurs, which cannot be detected by the ordinary protective apparatus, or which requires considerable time delay to enter into a protective operation.
The inventor and others are planning to construct a large capacity fuel cell power supply of 5-10 MW. This is expected to present the following problems of:
(1) Detecting overload and short-circuit failure in a single or system interconnection operation of the fuel cell power supply, and detecting the regenerative power when inverter failure occurs in the system operation.
(2) The capacity of the switch for opening the output circuit of the fuel cell 1 in a stable and reliable manner in response to the detection signal.
The problem (2) is important with regard to the second design and construction of the switch. This will be described in more detail. The output voltage from the fuel cell 1, as is known, is a direct voltage. Rough output values of the fuel cell 1 are shown in Table 1.
TABLE 1 ______________________________________ CAPACITY 5 MW 10 MW ______________________________________ RANGE OF APPROXIMATELY APPROXIMATELY DC VOLTAGE 1500-2200 V 1500-2200 V RATED APPROXIMATELY APPROXIMATELY CURRENT 4000 A 8000 A INFERRED APPROXIMATELY APPROXIMATELY SHORT- 5000 A 10000 A CIRCUIT CURRENT ______________________________________
The maximum capacity of ordinary direct current breakers for turning on and off the direct current is DC 1500 V/4000 A (circuit-breakers of the other companies are at similar levels). The DC breaker reduces its breaking capacity as the voltage increases until it reaches the point where it cannot break the current.
To maintain the current capacity, strategy such as using two 4000 A DC breakers connected in a parallel fashion can be taken. However, such current switches that can serve for the 5 MW-10 MW fuel cell power supply have not yet been developed.